Pulsed scavenging of two cycle engines as obtained by crankcase scavenging has recognized advantages but requires special lubrication systems not favored for automotive use. Blower scavenging while well developed in two cycle diesel engines does not provide the pulsed flow of crankcase scavenging.
One of the features of most crankcase-scavenged two-stroke-cycle engines is a dry-sump design. This prohibits the use of inexpensive, tough journal-type bearings and, for multi-cylinder configurations, entails the further complication of splitting the crankshaft or the roller-type bearings located between cylinders. Split roller bearings are also needed for all of the connecting rods at the crankshaft. If crankcase compression is replaced by a blower, a conventional wet-sump crankcase with journal bearings becomes practicable.
External blowers, such as the roots type, are widely used to provide scavenge air for diesel engines. Examples are the uniflow Detroit and Electro-Motive diesel engines. Diesel engines are generally unthrottled and the goal is to achieve optimum scavenging effectiveness. Pumping systems for diesel engines generally provide a fairly constant delivery pressure to the engine cylinders throughout the scavenge period. This reduces the punping work required compared to a crankcase scavenged engine.
In contrast to the uniform pressure in a blown engine, the pressure in a crankcase-scavenged engine varies significantly during the delivery period. This produces a strong pulse of flow into the combustion chamber as the intake ports are being uncovered. At lower speeds, this initial pulse may be driven by a pressure of about one-half atmosphere in excess of ambient pressure and be dissipated before the piston has uncovered half of the intake port area. This pulse has a strong influence on the scavenge flow development inside the combustion chamber. It can strongly influence the gradient of fresh charge concentration within the cylinder. At part-load operation the fresh charge may only represent one-third or less of the cylinder contents, the rest being residual products, so the gradient of fresh charge relative to the location of the fuel injector during the injection period and relative to the spark plug location at the time of spark discharge can be critical to the combustion process. Experimental results showed that much inferior performance was achieved from an engine converted from crankcase-compression operation to a uniform pressure delivery system. Studies for variable and constant pressure air delivery systems found very significant alterations of the gradients of fresh charge concentration between the variable and constant pressure systems.